Bump README python requirement to 2.6.0.

The code already checks for python 2.6.

doc/README

 
 Requirements
 ============
-- Python (>= 2.4.4)
+- Python (>= 2.6.0)
 - Git (>= 1.5.2.4)
 - Gnuplot (>= 4.0.0)
 - a git repository (bare clone will work as well)